Recently, there has been an increased concern about protecting the environment. Thus, carbon dioxide emissions have been reduced in various industries. In the automobile industry, the early introduction of Hybrid Electric Vehicles (HEV), Electric Vehicles (EV) and fuel cell vehicles seeks to reduce carbon dioxide emissions. To introduce such vehicles at an earlier stage, a high performance secondary battery must be developed. In recent years, the focus has been directed to a stack-type bipolar secondary battery, which can achieve high energy. High output densities have been contemplated for the secondary battery.
A conventional bipolar secondary battery includes a battery element, wherein a plurality of bipolar electrodes are stacked using an electrolyte layer interposed therebetween. Such a battery also includes an outer material for surrounding and sealing the entire battery element, as well as a terminal projecting from the outer material to the outside for extracting a current. The bipolar electrode forms a cathode by forming a cathode active material layer at one surface of a collector and an anode by forming an anode active material layer at the other surface of the collector. Further, a unit cell layer is formed by sequentially stacking the cathode active material layer, the electrolyte layer and the anode active material layer. Such a unit cell layer is inserted between a pair of the collectors. As for a bipolar battery stack, since the current flows along a direction of stacking the bipolar electrodes in the battery element (i.e., a thickness direction of the battery), a path of the current is short. Thus, current loss is low.
Japanese Laid-Open Patent Publication No. 2006-073772 (“Patent Document 1”) discloses a laminate bipolar secondary battery as an example of the bipolar secondary battery. According to the bipolar secondary battery disclosed therein, an end plate is positioned at both end surfaces in a stacking direction of the bipolar secondary battery. Further, the bipolar secondary battery is pressed by such end plate from the stacking direction of the bipolar secondary battery. By pressing the bipolar secondary battery as above, there can be obtained a bipolar secondary battery with high energy density and high output density.